This invention relates to arrangements for actuating airbags which include an airbag control unit, an actuating switch for activating and deactivating the airbag, and a visual display device.
Motor vehicles are increasingly being provided with airbags which expand in the event of a collision to protect the driver or passenger against an impact with hard parts of the vehicle structure. However, the airbag should be capable of being switched off, first so that it will not be released when a passenger seat is unoccupied, and second in order not to injure the driver or passenger in certain situations, for example when a child seat is placed on the passenger seat.
For this purpose, a mechanical switch is inserted in the circuit between the control unit and the airbag by which the airbag control unit can be electrically separated from the airbag. Such a switch is disclosed for example in U.S. Pat. No. 5,641,952. Simultaneously with the electrical separation, a visual display, for example in LED form, is energized so that the deactivation of the airbag is visually indicated. The electrical separation caused by actuation of the switch produces an open condition at the terminals of the airbag control unit and this might cause a malfunction since the airbag control unit cannot distinguish between an open condition due to a switching operation and a line interruption. Therefore, a safety resistor is inserted following the actuating switch in parallel with the airbag. This safety resistor has the same resistance value as the resistance of the firing pellet for the airbag. The actuating switch is arranged to complete the electric switch circuit either by way of the airbag or by way of the safety resistor so that the airbag control unit is always presented with the same resistive load.
Furthermore, there are conventional airbag control units that are electrically connected to the airbag in a permanent manner and have a visual display associated directly with the airbag control unit. In such arrangements, the airbag control unit is connected through two resistances in series to a source of operating voltage. The actuating switch is arranged in parallel with one of the resistances so that that resistance is short-circuited when the switch is closed. The resulting change in total resistance is detected by the airbag control unit by the corresponding change of current, causing the visual display to be turned on. A defect in the visual display device can also be detected by the airbag control unit by a change of current despite an unchanged electrical connection. In that case, the airbag control unit will not generate a firing signal for the airbag. However, this very advanced concept requires a completely redesigned airbag control unit so that considerable retrofitting would be required to meet the same safety standards in older motor vehicles.